The present invention relates to a fuel channel box for forming a fuel assembly for a boiling water reactor and, more particularly, to a fuel channel box resistant to flexural deflection due to exposure to neutron irradiation, and a method of manufacturing such a fuel channel box.
A method of uniformizing the exposure dose rates of fuel assemblies by changing the positions of the fuel assemblies in a reactor has been studied to prevent the flexural deflection of fuel channel boxes due to irregular neutron exposure dose rates. Hereinafter, flexural deflection due to exposure to neutron irradiation will be referred to as xe2x80x9cirradiated bowxe2x80x9d. However, irradiated bow has not been prevented yet. Irradiated bow reduces a space for the movement of a control rod and changes fission reactivity. Thus, irradiated bow is a principal factor that shortens the life of the fuel channel box.
Techniques for randomizing the orientations of crystal grains to prevent irradiated bow are disclosed in Japanese Patent Laid-open Nos. Sho 59-229475, Sho 62-200286, Hei 5-17837, Hei 5-80170 and Hei 9-249927.
The technique disclosed in Japanese Patent Laid-open No. Hei 9-249927 restrains the upper and the lower end of a heating coil from movement in all lateral directions by rollers so that clearance between the heating coil and a fuel channel box may not change and heats the fuel channel box at a temperature uniformly over the entire width of the walls of the fuel channel box.
However, the fuel channel box is a welded structure originally warped and twisted with respect to a longitudinal direction within tolerances. Therefore, the clearance between the heating coil and the fuel channel box changes, causing temperature difference between the opposite walls of the fuel channel box. The temperature difference between the opposite walls causes difference in orientation (FL value) with respect to the longitudinal direction (rolling direction) between the opposite walls, which results in flexural deflection.
A method of reducing the orientation difference AFL between the orientations of the opposite walls subjects the fuel channel box to heat treatment a plurality of times, which, however, reduces production efficiency.
Japanese Patent Laid-open No. Sho 59-16956 and U.S. Pat. No. 4,548,657 disclose techniques that heat a pair of adjacent walls of a fuel channel box at a temperature in a temperature range including xcex2-phase temperatures and heat other walls at a temperature in a temperature range including xcex1+xcex2-phase temperatures to control deflecting direction. These techniques take nothing about deflection due to temperature difference between the opposite walls into account.
It is an object of the present invention to provide a fuel channel box that is not subject to significant irradiated bow even if a bow is produced therein originally when the same is manufactured and can be manufactured at a high production efficiency, and a method of manufacturing such a fuel channel box.
According to a first aspect of the present invention, distance between a heating coil and a fuel channel box of a Zr alloy is controlled so that temperature difference between the opposite walls of the fuel channel box is reduced when heating the fuel channel box at a temperature in a temperature range including xcex2-phase temperatures for beta-quench treatment. The temperature range including xcex2-phase temperatures is a temperature range beyond 980xc2x0 C., preferably a temperature range of 1050 to 1350xc2x0 C., more preferably, a temperature range of 1050 to 1200xc2x0 C.
According to another aspect of the present invention, a fuel channel box of a Zr alloy is heated at a temperature in a temperature range including xcex2-phase temperatures so that temperature difference between the opposite walls of the fuel channel box is 50xc2x0 C. or below, preferably, 10xc2x0 C. or below.
The fuel channel box treated for beta-quench treatment has a characteristic to limit a flexural deflection due to exposure to neutron irradiation at a burnup of 55 GWd/t to 1.8 mm or below. Preferably, the fuel channel box of the present invention has a characteristic to limit a flexural deflection due to exposure to neutron irradiation at a burnup of 70 GWd/t to 1.6 mm or below.